A self-aligned process for forming a silicide layer is popularly used in a semiconductor manufacturing process, and especially commonly used in an MOS process. The self-aligned process can be advantageously used to produce a self-aligned silicide layer, or "salicide" for short, of low resistivity on the surface of a silicon or a polysilicon layer. Moreover, no petty photolithography procedure is required in this process. For a VLSI process to produce a device of a reduced size and/or a deep-submicron level, such a contact metallization process has great potentiality.
Please refer to FIGS. 1A.about.1F which schematically show a conventional process for forming a self-aligned silicide in an MOS manufacturing process. In many cases, titanium is used as the metal for forming the self-aligned silicide. FIG. 1A schematically shows the formation of a polysilicon layer 13 over a silicon substrate 10 having been formed thereon a field oxide 11 and a gate oxide 12. FIG. 1B schematically shows the step of defining a gate 14 for the structure of FIG. 1A. FIG. 1C schematically shows the deposition of an oxide layer which is further etched to form a spacer 15 on the structure of FIG. 1B. This deposition procedure can be a chemical vapor deposition (CVD) process. FIG. 1D schematically shows the deposition of a titanium metal layer 16 on the resulting structure of FIG. 1C. In this procedure, the metal layer 16 can be deposited by a sputtering process. FIG. 1E schematically shows the formation of a titanium silicide (TiSi.sub.2) layer 17 of C49 phase. The layer 17 is formed by a rapid thermal process (RTP), wherein portions of the titanium metal 16 react with the silicon 10 of the source and drain regions, and the polysilicon 14 of the gate region thereunder at a high temperature of 650.degree. C. with the introduction of a nitrogen gas. FIG. 1F schematically shows the transformation of the undesired C49-phase TiSi.sub.2 layer 17 into a desired C54-phase TiSi.sub.2 layer 18 which has a lower resistivity. Before the formation of the TiSi.sub.2 layer 18, the primitive titanium metal 16 which does not react with silicon or polysilicon, or the titanium nitride 161 produced by the reaction between the titanium metal and the introduced nitrogen are removed by selectively etching. Then, another rapid thermal process is performed at an even higher temperature of 825.degree. C. with the introduction of nitrogen to form the TiSi.sub.2 layer 18 so as to complete the formation of the salicide of the gate in the MOS manufacturing process.
In the self-aligned step shown in FIG. 1E of the conventional process, silicon atoms of the silicon 10 of the source and drain regions, and the polysilicon 14 of the gate region are likely to diffuse along the interface 19 (FIG. 2) between the unreacted titanium metal layer 16 and the spacer 15 due to the high temperature of the thermal process. As such, referring to FIG. 2, the width W of the spacer 15 should be enlarged to assure of enough length the spacer between the silicide 171 in the gate region and the silicide 172 in the source/drain region, thereby preventing the reaction between the titanium metal and the diffusing silicon atoms to cause the lateral growth of the silicide. As known to those skilled in the art, excessive lateral growth of the silicide takes a risk of short circuit, and seriously influences the yield of the process. Unfortunately, the enlargement of the spacer width does not comply with the current requirement in size reduction and may degrade the device. For example, the width of the spacer has certain effect on the domain of the lightly doped drain (LDD), and should be at a preferably specific value.
In order to avoid the above problems, a technique is proposed by Y. S. Lou and C. Y. Wu, and disclosed in a treatise entitled, "Lateral Titanium Silicide Growth and Its Suppression Using the Amorphous Si/Ti Bilayer Structure", Solid State Electronics, Vol. 38, pp. 715.about.720, 1995. According to this technique, an amorphous Si layer is used to isolate the titanium from the external oxygen impurities, and closely monitoring on the process conditions is performed to inhibit the adverse effect of the internal oxygen impurities on the growth of the lateral titanium silicide. This technique does have prominent effect on the suppression of the lateral growth of titanium silicide if the entering of the oxygen impurities into the titanium metal in the process is precisely controlled. Unfortunately, titanium is a good oxygen-gettering metal so that the isolation of the titanium metal from the oxygen impurities will be difficult. If the result described in the treatise is to be achieved, the cost for the equipment and the process control will be extremely high.